1. Technical Field
The embodiments herein generally relate to radio frequency (RF) technologies, and, more particularly, to filtering undesirable RF signals in a RF network using filters.
2. Description of the Related Art
The spectrum input to RF devices typically includes a large number of undesired signals in addition to the desired band of interest. Such interferences can be very large, possibly causing intermodulation distortion, desensitization, cross-band modulation, and oscillator pulling, among other undesirable effects. Most typical RF receivers require a band-limiting filter at their input to eliminate or reduce such interferences. These filters typically require very high selectivity (that is, a very narrow passband relative to the filter center frequency). In certain wide-band applications, these filters must move to track the desired channel. Typically, such “tracking” filters must be very carefully tuned or they may unintentionally attenuate the desired signal.
There are generally two conventional approaches to RF filtering. In applications where tracking is not required, an off-chip resonator such as a surface acoustic wave (SAW) filter is employed. The benefit of these filters is excellent selectivity and accurate passband location. However, the disadvantages are twofold. First, these filters generally have approximately 2 dB loss in their passband. This translates to an additional 2 dB of noise figure (NF) and thus directly affects the minimum possible sensitivity of the system. Second, these filters invariably add significant cost to the bill of material (BOM) and generally increases the circuit board area. For tracking applications, a tuning element such as a p-type intrinsic, n-type diode (PIN diode) is used to tune the resonance of a tank or similar resonant circuit. While this approach manages to provide a tunable filtering, it generally suffers from poor stopband attenuation and less passband frequency accuracy than SAW filters. Furthermore, these filters are off-chip, and again impact BOM costs. Even in the case of a tracking filter or an active notch filter, factory calibration/tuning is generally required (i.e., leading to more cost and complexity of implementation).
Accordingly, there remains a need for a high Q factor (high-Q) filter which does not require calibration and which can track the local oscillator (LO) signal and achieves large stopband attenuation.